1. Field of the Invention
The present invention relates to an electrophotographic transfer paper for use in a dry indirect electrophotographic full-color or monochromatic duplicating machine, printer, etc. which exhibits a high paper gloss and excellent running properties, particularly in a high humidity and temperature atmosphere.
2. Description of the Related Art
In recent years, the trend of more duplicating machines and printers is to be provided in a multi-color system, operate at a higher speed and provide a higher image quality. Also, in the art of on-demand publications, color duplicating machines or printers have been made able to meet the demand for easier publication of copies in a smaller circulation. Thus, there has been a marked trend toward the use of duplicating machines or printers in publishing this kind of printed matters.
A coated paper having a high white paper glossiness commonly known as a glossy coated paper is normally obtained by applying a particulate pigment having an average particle diameter of not more than 2 .mu.m to a substrate in an amount of not less F than 10 g/m.sup.2 for each side by means of any of various coaters, and then calendering the coated material to smooth the surface thereof. Such a coated paper having the high white paper glossiness has been normally used in the art of commercial printing. Also in duplicating machines and printers, the foregoing coated paper has been used more occasionally instead of conventional PC papers or printer papers to obtain an image which is sharper than ever.
In the operation of the duplicating machines or the printers, a stack of papers is placed on a tray disposed inside the main body thereof or a manual tray disposed outside the main body, and then fed into the printing zone one sheet by one sheet. However, when fed in a high humidity atmosphere as in rainy season, the printing coated paper having the high white paper glossiness can be subject to so-called misfeeding, that is, two sheets of the coated paper can be stuck to each other and thus cannot be fed. This is because the coefficient of friction between the printing coated papers in stack becomes very large in the high humidity atmosphere. Such a coated paper normally has a higher smoothness than other coated papers for use in duplicating machines or printers. Because of the presence of a coating layer, such a coated paper also has a low water absorptivity. Therefore, it is thought that the water content attached to a surface of the coating layer is not easily absorbed by the coating layer but adsorbed by a surface of the coated paper in the high humidity atmosphere. The smoother the coating layer is, the more easily this excessive adsorbed water on the surface of one coated paper can adhere to the surface of another. This is probably because the surface tension and hydrogen bond of the adsorbed water cause the papers to be stuck to each other, raising the coefficient of static friction therebetween and hence preventing the paper from running.
As a method for improving the running properties of coated paper there is proposed a method disclosed in JP-A-62-198875 involving an adjustment of the surface roughness of coated layer and the standard deviation of the coefficient of static friction between papers to not more than 2.0 .mu.m and not more than 0.05, respectively (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, this proposal has no recognition that water present between papers causes a poor running and the poor running is also due to the surface tension of water. Further, a method disclosed in JP-A-8-171226 which eliminates the poor running in the high humidity atmosphere by adjusting the smoothness and the air permeability of coated paper measured by Oken type testers to a range of from 60 to 300 seconds and a range of from 10 to 90 seconds, respectively, has been proposed. Regrettably, under these conditions, a coated paper having a white paper glossiness of not less than 55% cannot be produced.